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Laser hazard distance calculator

If you know the power, divergence and wavelength (precise color) of a visible, continuous wave laser, you can use the online calculator below to determine the eye hazard distances NOHD and ED50, and the Federal Aviation Administration visual interference hazard distances SZED, CZED and LFZED.

This calculator is valid only for lasers emitting visible (400-700 nanometers), continuous wave (CW) laser light over long distances. It assumes an unwanted exposure where a person moves and/or blinks within 1/4 second to avoid the light; this is a standard assumption within the laser safety field.

Enter the laser’s power in milliwatts.If you know the power in watts, multiply the watts by 1000 to get milliwatts. For example, for a 1.5 watt laser, enter 1500; for a 40 watt laser enter 40000.

Milliradians (divergence)mrad

If you don’t know the beam divergence, use 1 milliradian for lasers under 500 milliwatts in power, and 1.5 milliradians for lasers 500 milliwatts and above.

Visual Correction Factor (between 0 and 1)

Below you can find the FAA Visual Correction Factors for selected laser wavelengths. For example, if your laser is the common 532 nm green, you would enter 0.8802 as the VCF. If you don’t know the laser’s wavelength, or if you want the safest, most conservative calculations, enter 1 as the VCF — this will give the longest hazard distances.

Distance units

Results are below, in black. The numbers are in the distance unit selected above, to the nearest tenth of a unit (one decimal place). The results are continuously updated as you enter new information — you do not have to click a “=‘ or “Calculate” button.

Eye hazard results

These distances depend only on the power and divergence of the laser.

Nominal Ocular Hazard Distance.0

(
32.8
/
[mrad]
)
*
(
sqrt
(
0.5
*
[mw]
)
)
*
[units]

The Nominal Ocular Hazard Distance is where the laser beam has spread enough so that the light entering the eye is considered safe. (Specifically, at this distance there is “a negligible probability of damage” according to the laser safety standard ANSI Z136.1). Beyond the NOHD, the laser beam irradiance falls below the Maximum Permissible Exposure.

ED50 distance.0

(
(
32.8
/
[mrad]
)
*
(
sqrt
(
0.5
*
[mw]
)
)
/
3.16
)
*
[units]

At the ED50 distance, there is roughly a 50-50 chance that a fixed laser beam aimed into an unmoving eye under laboratory conditions will cause the smallest medically detectable change to the retina.

The ED50 distance is about 1/3 of the NOHD. The extra space between the ED50 distance and the NOHD gives an added “safety margin” or “reduction factor.” Beyond the NOHD, the laser light is considered unlikely to cause any eye injury.

Being exposed to laser light within the NOHD does NOT mean that a person will automatically receive an eye injury, or even is likely to have an injury. The NOHD is a “nominal” hazard distance, not an actual hazard distance. The closer the person is to the laser, the greater the chance of an injury, as shown by the colors above.

Visual interference results

These distances depend not only on the laser’s power and divergence, but also on the Visual Correction Factor of the laser’s wavelength. That’s because the human eye is more sensitive to greens and yellows, and less sensitive to blues and reds.

These three visual interference distances are used by the U.S. FAA to determine how far a laser beam can travel before it falls below the irradiance limits in a Sensitive, Critical or Laser Free zone.

Sensitive Zone Exposure Distance (flashblindness).0

(
(
32.8
/
[mrad]
)
*
(
sqrt
(
12.7
*
[mw]
*
[vcf]
)
)
)
*
[units]

Beyond the SZED, the laser light is not expected to cause temporary flash blindness or an afterimage.

Beyond the LFZED, the laser light is not expected to be distracting. It will not be brighter than other city or airport lights seen outside the cockpit window.

Caution — when the VCF is less than 0.0395

If any of the visual interference distance values — SZED, CZED or LFZED — are less than the NOHD, then use the NOHD value instead. This is to avoid cases such as a blue or red beam which appears so dim to the eye that it has a short visual interference distance, but can still be an eye hazard to a greater distance. The goal is to not allow a person to be exposed at a distance shorter than the NOHD.

Take for example a deep blue 445 nm laser. Enter the VCF of 0.0305 in the calculator. The SZED will be less than the NOHD, no matter what the power or divergence. If you were filling out the FAA Advisory Circular 70-1 “Laser Configuration Worksheet,” you would be required to enter “Less than NOHD” in the SZED space.